But it will supplement finger testing, providing the more frequent blood monitoring that can help keep diabetics healthier, the FDA said. Better, it sounds an alarm if blood sugar hits dangerous levels -- possibly life-saving if glucose plummets while they sleep.

To use the prescription-only GlucoWatch, patients slide a thin plastic sensor onto the watch's back each time they strap it on. Small electric currents extract a tiny portion of glucose from fluid in skin cells instead of blood, measuring it every 20 minutes for 12 hours.

So far, the device is only for adults, the FDA stressed. That will greatly disappoint many parents of child diabetics, who particularly struggle with those painful finger-prick tests and are anxiously awaiting painless monitoring.

But not only has Cygnus not yet studied the GlucoWatch in children, doctors simply don't know if glucose measured in skin cell fluid correlates to blood measurements in children like it does in adults. So the FDA will watch closely to ensure it is properly prescribed just to adults.

California-based Cygnus did not immediately say how soon the watch would start selling, or its price. But physicians must be trained to use it, and then train patients who must accurately answer a quiz before Cygnus can ship them a watch.

Some 16 million Americans have diabetes, meaning their bodies cannot properly regulate blood sugar, or glucose. They check their levels by pricking a finger and placing a drop of blood on reactive strips.

Doctors urge those tests be done frequently, four to eight times a day, because they can help patients better control diabetes and thus lower their chances of debilitating complications such as blindness, kidney disease and nerve damage.

But these fingerstick tests are painful and inconvenient, leading the average patient to test only twice a day.

Even those who frequently test cannot know if glucose soars or drops between testing or during sleep. Indeed, many diabetics who become hypoglycemic at night set alarm clocks to wake them every few hours for testing and treatment.

But never decide to use insulin based on a GlucoWatch reading without first double-checking with a fingerstick test, the FDA warned. While the GlucoWatch generally is as good as blood tests, a quarter of the time GlucoWatch readings can differ from blood tests by about 30 percent.

That could be a problem, particularly in detecting hypoglycemia, or low blood sugar. But Cygnus says patients can program the GlucoWatch to sound an alarm before glucose plummets to dangerous levels, giving time for a blood test.

Also, the watch won't measure if the patient's arm becomes too sweaty and is less effective at detecting very low glucose than very high levels, the FDA cautioned.